Properties of Different Coronal Streamers

Abstract

In this paper a summary of results from coordinated observations of streamers acquired by the SOHO Coronal Diagnostic Spectrometer (CDS) and Ultra Violet Coronograph Spectrometer (UVCS) experiments is presented (Parenti et al. 2000). Data from different altitudes within the solar corona were taken, with the purpose of determining their physical parameters – densities, electron temperatures and element abundances – and their changes over the altitude range between 1.02 and 1.6 R⊙. Further UVCS streamer data, taken about two months later are used to compare with the behaviour seen in two different streamers. The streamers resulted to have a different behaviour in temperature and density. The low corona, covered by CDS data, shows little evidence of abundance variation with respect to photospheric values, while, at UVCS altitudes, a depletion of all element abundances is clearly evident.

Physical parameters of a mid-latitude streamer during the declining phase of the solar cycle

“…These density values are systematically lower than those derived by LASCO C2 pB data inversion, in some cases even a factor of five lower. At low heliocentric distances, they are in good agreement with the densities obtained by the method of Parenti et al (2000), with an O vi disc intensity input equal to the quiet-Sun value reported by Vernazza & Reeves (1978).…”

“…Moreover, the disc intensity of the O vi 1032 Å line was used in Eq. (8) of Parenti et al (2000) to determine the electron density at heliocentric distances lower than 2.25 R . As for the Lyα line, we adopted the average chomospheric profile reported by Gouttebroze et al (1978), which agrees, within the uncertainties, with the full-disc Lyα intensities measured by the spectrometer SOLSTICE (SOLar STellar Irradiance Comparison Experiment) on the UARS satellite (see Woods et al 2000) during the week of our observations.…”

“…The agreement of the emission-line intensities, profiles, and intensity ratios synthesized from the resulting empirical model of the streamer with the spectral data reported in Figs. 3−7 appears remarkable, well within the data uncertainties, particularly in the case of the O vi line intensity ratio. Figure 8 shows the radial profiles of the electron density along the central region of the mid-latitude streamer monitored during our week of observations, derived from the inversion of LASCO C2 pB data relevant to the different days of observation, together with the density values obtained at lower heliocentric distances from the UVCS data collected on May 17 (see Table 1), adopting the method proposed by Parenti et al (2000).…”

“…(8) of Parenti et al (2000) were obtained assuming an electron temperature equal to 1.1 × 10 6 K (Fineschi et al 1998) over all the 1.6−2.6 R range of heliocentric distances. Different symbols refer to different values of the O vi 1032 Å disc intensity adopted in the relationship from which densities have been derived, respectively 116, 305, and 610 erg cm −2 sr −1 s −1 .…”

“…The inversion of such pB data (see, e.g., Hayes et al 2001) provided daily electron density maps relevant to the portion of extended corona observed in UV, in the 2.25 to 6 R range of heliocentric distance. We also determined the electron density (N e ) below 2.25 R by adopting the method described by Parenti et al (2000), based on the dependence on N e of the ratio of the radiative and collisional components of the O vi 1032 Å line. This technique works under the condition of a nearly static plasma, as is the case of the examined streamer below 2.25 R , so that the Doppler dimming effect on the radiatively excited component of the O vi lines can be considered negligible.…”